Jim is taking his realism dangerously far. Literally. As in, unless they walked at least 100km, they are in serious danger.

Fast-talking the GM is fun, but you should be careful not to fast-talk your way into dying.

— aurilee

Just a quick double check. That's Jim's character. Jim's the geologist. So his knowledge of what he's talking about is probably pretty darn good, right? Except that he declared how big the explosion is on his own, without the GM's comment about how big the explosion was.

Actually, that is consistent with his character isn't it?

The shockwave/dust storm scene in panel two, now that's pretty. A nice beautiful perfect circle. Adam and Jamie would be proud to have made something like that.

How far did they walk anyways? You might think that the mountains that they walked around would protect them by providing some shelter from the storm, but a look at real-world volcanic eruptions will quickly show you that an eruption the size of a volcano usually doesn't care about pesky little mountains.

— Keybounce

Transcript

K-2SO: Won't this huge explosion cause like a nuclear winter on Tatooine or something?Kyle: Oh, no, it's roughly the same volume of ejecta as a large volcanic eruption. It'll make the sunsets purple all over the planet for a few weeks, but nothing more serious than that.Chirrut: Except they blasted a whole bunch of data crystals into the sky.Kyle: Hmm. That could cause some interesting refraction effects with the insolation. People probably shouldn't look at the sky for a few days - they could get radiation burns on their retinas.Chirrut: Oh, I can live with that.Kyle: Of course, in the first minute or so the water table will boil and blow the entire surface layer sky high within a hundred kilometre radius.GM: Well... not necessarily. It could be quite local.Kyle: Don't be ridiculous! This may be fantasy, but there have to be limits. Don't break my suspension of disbelief.
{beat, explosion spreads across the surface of Tatooine...}Cassian: So...Cassian: How far did we walk today?